Conforming dies for the repetitive stamping of large numbers of identical sheet metal parts (e.g., automobile body parts) are typically made of high strength tool steel because of its rigidity and durability. Such dies are expensive and require appreciable manufacturing time because a large casting must first be made, and then complex forming surfaces accurately machined therein.
Automobile manufacturers have made stamping tools for making prototype srampings from lower strength zinc alloys such as Kirksite, or from mass castable polymeric tools. One family of materials commonly used in such polymeric tools is epoxy resins that can be cast to size into high strength tools. For example, Dearlove et al. U.S. Pat. No. 4,423,094 discloses a tough durable bisphenol A epoxy composition for making sheet metal stamping dies. Moreover, Wang et al. U.S. Pat. No. 4,920,161 discloses a room-temperature-cured cast-to-size epoxy tooling material having high loadings of specific particle sizes of inert fillers (e.g., silicon carbide and silica) have high tensile strengths. Others have proposed room temperature curable epoxy resins systems such as those based on bisphenol A and aromatic amine catalysts. Still further, Dearlove et al. U.S. Pat. No. 5,280,053 discloses a readily machinable, highly filled (i.e., with iron powder and glass fibers) tooling epoxy. Tools made from such materials have a limited useful life typically less than 5000 parts.
In the making of such polymeric tools a pattern or "casting aid" is machined or otherwise constructed to the desired configuration of the sheet metal part to be made, and the epoxy then cast against the pattern to form the desired shape in the epoxy. Patterns for this purpose typically comprise natural wood, epoxy, urethane, plaster, clay, sheet wax, steel and aluminum.
It would be economically attractive to make polymeric dies that are more durable so that they could be used for low volume production runs rather than just for a few prototype parts. It has heretofore been proposed to increase the durability and useful life of the forming surface of polymeric dies by metallizing such surface with low melting point metals such as zinc. To this end, the pattern is (1) first coated with a high temperature black spray paint such as consumers commonly use to paint outdoor cooking grills (e.g., a methyl phenyl silicone-polyester based paint such as TAFA 4140 Black Paint), (2) a coating of PVA is applied over the black paint, (3) zinc is then arc-sprayed atop the PVA, and finally (4) an appropriate epoxy is cast against the zinc so as to adhere thereto and serve as a backup for the zinc. The arc-spray technique yields a rough exterior surface comprising a plethora of peaks and valleys to which the epoxy readily adheres.
Unfortunately, low melting point (i.e., &lt;ca. 400.degree. C.) metals like zinc are likewise relatively soft, and accordingly do not provide the durability and extended useful life that harder metals would. Harder metals, however, typically have a much higher melting point, and attempts to arc-spray such harder metals onto the patterns prepared in the same manner as they are to receive zinc has resulted not only in a degradation of the pattern's surface, but a high incidence of droplets of the metal actually bouncing off of the pattern surface rather than adhering thereto both resulting from the molten metal burning through the paint and PVA coatings. The net effect is that attempts to arc-spray higher temperature metals such as silicon bronze (MP 1000.degree. C.), steel (MP 1500.degree. C.), molybdenum (MP 2600.degree. C.), copper (MP 1083.degree. C.), nickel (MP 1455.degree. C.) and aluminum (MP 660.degree. C.) onto the pattern have heretofore been ineffective to produce quality epoxy tools.
Accordingly, it is an object of the present invention to provide a process for producing a metallized epoxy tool including a unique pretreatment for the tool-making pattern which permits metallization thereof with harder, higher melting point metals than zinc.
This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.